Osteoarthritis (OA) is the most prevalent joint disease affecting most of the elderly, imposing a major socio- economic burden on society. The hallmark of the disease is the breakdown of articular cartilage by elevated proteinase activities that degrade major extracellular matrix molecules, aggrecans and collagens, but currently there are no effective treatments for OA, except joint replacement surgery. Our long-term objective is to find ways to specifically inhibit these proteinases and apply them for clinical intervention in OA. The key enzymes involved are matrix metalloproteinases (MMPs) and adamalysins with thrombospondin motif (ADAMTSs). Their activities are regulated by the endogenous tissue inhibitors of metalloproteinases (TIMPs). Our recent studies have shown that TIMP-3 variants that selectively inhibit aggrecanases prevent the progression of OA in an animal model, whereas wild-type TIMP-3 that broadly inhibits MMPs and ADAMTSs was not effective. This suggests that highly selective inhibitors are essential for therapeutic development. To achieve this goal two major aims are proposed. The first is to understand the molecular basis for selectivity in our TIMP-3 variants using biochemical, biophysical and structural methods, and to use this to further develop highly discriminating inhibitors of metalloproteinases. Specifically, we will rationally design TIMP-3 variants that differentially inhibit the main two aggrecanases, ADAMTS-4 and -5, and screen human antibody phage display libraries for exosite inhibitors of aggrecanases and collagenases. The prototype exosite inhibitors will be further refined by mutation. The mechanism by which calcium pentosan polysulfate (CaPPS), an exosite inhibitor of aggrecanases, enhances the interactions between TIMP-3 and aggrecanases will be also elucidated. The second major aim is to evaluate the efficacy of available and newly developed metalloproteinase inhibitors using in vitro and in vivo models of OA and to validate target enzymes in human OA using these inhibitors. To this end, we will continue characterizing TIMP-3 and [-1A]TIMP-3 transgenic mice to delineate the molecular basis of their differential effects on blocking OA progression by analyzing extracellular matrix components protected by these inhibitors using a combination of immunological and proteomic approaches. Newly developed exosite inhibitors and TIMP variants will be tested for their efficacy to protect cartilage from degradation using an in vitro cartilage explant culture system and the in vivo mouse model of OA induced by medial meniscotibial ligament transaction. Effective inhibitors will be also tested in human cartilage in culture to further validate their efficacy. Using available inhibitors we will identify and characterize a novel TIMP-1- sensitive aggrecanase as it is considered as a potential therapeutic target in humans. Accomplishment of these goals will provide new principles for developing therapeutic interventions for OA.